I used Google and Facebook when first building the webapp because I simply didn't want to do the work of building an authentication system. It's lots of work to get right.
About a year ago I had a lot of users complain my site _only_ had social logins so I had to implement my own anyhow.
Now I have it, I don't see a lot of value in keeping the social logins around.
I just need to ask users to add a password to their account and they can login using email/password combo.